Thanks everyone for the replies! :D
I'm inclined to consider it just a GRV, but I'm not totally sure about it (that's why I'm asking :D).
The point is: the “previous error clause” exists because it allows the opponent to stop the action
before it leads to an illegal game state.
This is why, I think, we don't apply HCE in such situations: some of the fault lies with the opponent if he/she doesn't point out the error, so we don't allow him/her to look at the offender's hand (which would be a great advantage for him/her).
I think this scenario falls in this interpretation. If my opponent pays a wrong amount of mana, it doesn't matter if he/she draws 3, 4 or N cards: any draw is illegal and I'm responsible of that error as well.
I had a chance to stop him/her, but I didn't, so I should not be allowed to “destroy” his/her Hand.
It's the same when I cast
Think Twice and I say “I draw two cards” and my opponent says “ok!”. I don't consider it HCE for the same reasons. :)
Edited Jacopo Strati (Sept. 17, 2016 10:30:52 PM)